Barber Shop Trimming

There are different kinds of trimmers that barbers use. Most are intended to cut hair at certain lengths and have guards to ensure that appropriate lengths of hair are left intact. But there is also a trimmer that touches directly on the skin and is used to even out the edges and remove neck hair. My longstanding practice at the barber shop is to politely ask the barber to only use that trimmer behind my ear, i.e. on the back of my head and neck. That way, it will not remove my peyos. I never asked a she'eilah on this because it seems pretty clear cut. But every time I ask a barber to do it he seems surprised. Is this not common practice? Why not?